Laminates provide a way of strengthening frangible material, for example glass, so as to extend its uses and to render it safer to use in certain circumstances. Laminated glass products can be used for automotive and aircraft glazing, glass doors, balustrades, bulletproofing and many other uses where the glass product must be strong and/or shatterproof. A number of methods for producing such laminates have been disclosed (see, for example, U.S. Pat. Nos. 5,268,049; 5,118,371; 4,724,023; 4,234,533; and 4,125,669). Conventionally, laminated glass is produced by forming a glass sheet assembly or stack which is made up of at least two flat sheets of glass with a layer of polymer adhesive laminating film sandwiched between adjacent sheets. In the event that the laminated glass is caused to break or crack in use, the function of the polymer adhesive laminating film is to hold the broken pieces of glass together. Bonding between the laminating film and the glass sheets is traditionally achieved using a combination of evacuation, pressing and heating.
The main problem encountered in the production of laminated glass is that air and/or moisture becomes trapped between the laminating film and the glass surfaces which can cause bubbling of the laminating film which is considered a defect which can render the glass unacceptable for use. Using prior art processes, the air is removed by diffusion or by dissolving in the film. Both processes are very slow, requiring long term post-lamination heating and/or the application of a high pressure cycle after lamination. The bigger the glass sheet, the longer the time that is required for removal of air from laminated glass. As a result, the productivity of such prior art processes is low and they require considerable capital expenditure for the necessary costly apparatus such as autoclaves.
Several prior art patents are directed to methods of laminating glass which are focused on allowing the air to escape during lamination. In U.S. Pat. No. 5,268,049, the glass sheets are spaced apart, and in the method described by U.S. Pat. No. 5,268,049, a liquid resin is used. In U.S. Pat. No. 4,234,533 the two sheets are held at an angle and in U.S. Pat. No. 5,118,371 the thickness of the laminating film gradually increases (or decreases) from the one side to the other side of the glass sheets. In U.S. Pat. No. 3,509,015, a method is described for producing laminated glass by sealing the periphery of two parallel glass sheets with pressure sensitive tape and forcing resinous material under pressure into the inter-sheet space. The resinous material is forced through a self-closing valve held in place with the tape while trapped air escapes through an aperture in the taped seam at the top of the cell. U.S. Pat. No. 4,125,669 describes a similar method in which two glass panes are sealed all around except for a filling opening and an aeration opening, and a binder material is introduced into the envelope thus formed in an amount calculated to exactly fill the envelope. Putty is applied to the openings just before emergence of the binder upon laying the filled envelope flat. In U.S. Pat. No. 7,063,760, the laminating film is applied to the first sheet of glass and then heated with microwave radiation to a bonding temperature and heated areas of the film are successively pressed to the glass sheet in a continuous manner to purge air from between the film and the first glass sheet. The pressed film areas are then cooled and subjected to a partial vacuum before a second glass sheet is positioned on the film. The film is then reheated with microwave radiation to a bonding temperature and thereafter cooled whereby an appropriate bond is obtained between the film and the second glass sheet to provide a glass lamination.
These solutions still require multi-pass operation, high energy consumption and, often, expensive equipment such as high pressure autoclaves which allow limited flexibility in their adaptation for use on various types of glass. Accordingly, there is a need in the art for a more flexible and less expensive method for laminating glass sheets which reduces energy consumption.